A bar code is a special optical sign system readable to an optical scanning device, which can be divided into 1-dimensional or two dimension bar code according to its spatial dimensions involved.
A code form for a 1-dimensional bar code is a combination of color bars and space bars with different widths along one direction, and the method and system for its identification are simple and thus is widely adopted in various fields. A typical 1-dimensional bar code includes UPS-bar code, EAN-bar code and 128-bar code. Since a 1-dimensional bar code contains little information, it is jointly used as a piece of index information together with a database.
A two dimension bar code can have two categories: a stack bar code and a rectangular bar code hereinafter referred to as a matrix code. The stack bar code is a two dimension bar code developed from the 1-dimensional bar code and is constituted through stacking multiple sets of 1-dimensional bar codes and in this regard, it contains a little more information than that a two dimension bar code does. A typical stack bar code such as PDF417 may store alphas up to 1K and could be higher if “jointing” signs have been accounted for. The defect of a stack bar code could be large in area, low in information intensity and thus is not suited for a small object. The coding principle and its configuration for a matrix code is entirely different from a 1-dimensional bar code or a stack code with the former adopting “a cell” or “a block”, hereinafter referred to as a matrix element, as a basic information unit. The binary optic characteristic of the matrix element represents a binary 0 or 1. A typical matrix code includes Data Matrix, MaxiCode and QR codes, etc. This kind of binary code has a major merit of high intensity of information, small area of code occupation and greater inclusion in printing and identifying due to its stronger error-correcting function provided through encoding.
To identifying a bar code normally requires adoption of a two dimension charge coupling device (CCD) as a scanner for an identifying device. During the identification, the light rays directed onto a heavily colored matrix element are absorbed but those directed onto a lightly colored matrix element are reflected back to the scanner. The scanner sequentially converts the intensity of the reflective light from each matrix element into a corresponding electrical signal and further into a digital signal by means of an analog to digital converter. A decoder translates these digital signals into message contents according to an algorithm.
Among these bar codes, in order for accurately locating each matrix elements, it is necessary to strictly synchronize the two mutually perpendicular scanning steps or otherwise the bar codes obtained through scanning will be distorted so as to affect the accuracy of the identification. This further leads to a very high requirement on the synchronism to the identifier and on the printing quality of a bar code, as well as expensive cost of the devices and the printing, and thus heavily block the popularization and the application of the two dimension bar code. This is the very reason the application of the two dimension bar code is only limited to high tech fields since its creation a decade ago.
To solve this problem in scanning synchronization, a special information unit for scanning synchronization, i.e. a synchronous information unit, is introduced into some rectangular bar codes. These units are disposed at a specific location where a rectangular bar code is located for embodying synchronous scanning along a longitudinal direction. To facilitating an identifying process, e.g. when in a Data Matrix bar code, the information synchronous unit is disposed at the two perpendicular neighboring borders of the bar code area. However these synchronous controlling information still can not lower any requirement on the scanner's synchronization. Taking a Data Matrix bar code as an example, in order for ensuring all coded information units in a scanned two dimension bar code to have identical size and a rectangular shape, it is required that the horizontal scanning of the scanner strictly synchronizes with its vertical scanning so as to ensure non-aberration and non-distortion of the image. This requires high accuracy in synchronization to the scanner and thus leads to a high cost to the identifying device.